1. Field of the Invention
This invention relates generally to pulse generators and is particularly concerned with pulse generator or pacemaker/defibrillator systems and methods for cardiac applications (cardiac rhythm management), as well as display devices and methods for set up and monitoring operation of external pacemakers/defibrillators.
2. Related Art
Pulse generators in general but in particular for cardiac applications usually generate an output of specific rate, amplitude and duration of the pulse. Amplitude refers either to a voltage (for instance 0 . . . 18 V) or a current (for instance, 0 . . . 25 mA) and is specified for a load or impedance range (for instance, 200 . . . 2,000Ω).
Pulse generators for cardiac applications are referred to as pacemakers. The impedance, or load, a pacemaker is facing is determined by the impedance of the lead wires and possible extensions thereof, the impedance of the heart tissue (myocardium, epicardium), and the impedances of the electrode-tissue interfaces (for a pair of electrodes). The impedance of the lead wires is usually of resistive nature and is generally a few ohms (for instance, approximately 10 ohms for a unipolar myocardial pacing wire with a length of 60 cm). The impedance of the heart tissue depends on the distance between the electrodes and also measures several tens or hundreds of ohms.
The impedance of the electrode-tissue interfaces is determined by their effective surface area. The impedance of an electrode-tissue interface of a unipolar myocardial pacing wire with an electrode length of 10 mm, for instance, may measure 100 ohms or more. Variations in strength of the electrode-tissue contact and manipulation of the pacing wire insulation may lead to an increase or decrease of the electrode-tissue impedance.
If the impedance of a stimulation system is outside the specified range, the pacemaker may not be able to maintain the desired stimulation voltage or current. It is thus of importance to the operator of a pacemaker to know the impedance the pacemaker is facing.
Temporary cardiac stimulation is the preferred method of treating temporary rhythm disturbances which can potentially follow cardiac surgery. In view of this risk, prior to chest closure after cardiac surgery, a pair of unipolar pacing wires or a single bipolar pacing wire is attached to, or sutured onto, the outside of the heart chamber (epicardium or myocardium) to allow for cardiac stimulation, for example to the right atrium and right and left ventricle. The other end of the pacing wires is fed through the chest outside the patient's body for connection to a temporary pacemaker (also referred to as external pulse generator, or EPG) directly or via extension cables. Post cardiac surgery when temporary cardiac stimulation is not needed any more, pacing wires are removed simply by pulling the wires out of the patient's chest.
Pacing wires, also known as heart wires, come in various configurations. A very important property is the means of fixing or attaching the wire onto the heart. The fixation needs to be designed in such a way that a wire stays securely at the intended location but can be removed easily post cardiac surgery. Non-traumatic wires simply attach to the heart with or without suturing. Pacing wires with a zig-zag fixation rely on bending a portion of the distal wire into a zig-zag shape. Other pacing wires are designed so that a portion of the distal plastic insulation is peeled off in order to form tines or wings which secure the position of the wire onto the heart muscle. The bare wire, i.e., the portion of the wire without insulation, acts as the active electrode for stimulation.
Obviously, there is a fine line between secure fixation and easy removal of the wire. In a few instances removal of pacing wires incorporating tines (or wings) for fixation reportedly caused bleeding and cardiac tamponade. After the pacing wire is attached to the heart muscle, the wire's cardiac needle and some portion of the wire is cut off. It is up to the discretion of the cardiac surgeon how much of the wire is cut off and whether the distal portion of the insulation including the tines (or wings) remains in place or not. Obviously, liberal manipulation of the distal end of the pacing wire can result in a significant increase of the length and surface area of the bare wire, thus changing the electrical properties of the stimulation system, which in turn may affect the performance of the external pacemaker and jeopardize stimulation therapy.